It's been said that there's a fine line between folk remedy and medicine, but we would argue that the line is in fact quite wide, and it's defined by a simple question: What can be scientifically proven to work? Unfortunately, that proof can take centuries, even millennia, to move beyond mere anecdote. Hippocrates advised pain and swelling sufferers in the fifth century BCE to chew willow bark, but it wasn't until fairly recently that we learned not only that it worked, but why — namely, that white willow contains salicin, a close chemical cousin to aspirin (acetylsalicylic acid) [source: Ehrlich].

Fast forward to 2015, when an old English recipe for an eye salve began making news for its promise as a weapon against methicillin-resistant Staphylococcus aureus, aka MRSA, the obstinate scourge of hospitals and nursing homes. The brew, which contains garlic, onion or leeks, wine and cow bile, was described 1,000 years ago in "Bald's Leechbook," an Anglo-Saxon collection of remedies now housed in the British Library. Remarkably, when researchers concocted the remedy, it not only affected MRSA — it eradicated it [sources: BBC, Rayner].

Some scholars believe that the early physicians who contributed to the book used a proto-scientific trial-and-error method and kept careful notes on which mixtures worked and which ones failed. Unfortunately, the MRSA killer had to spend a millennium forgotten among dubious tonics before its value was found again [sources: BBC, Rayner].